376 



METAZOAN PHYLA 



them from drying and also need to be safeguarded from mechanical 

 injuries. Certain coverings of the egg meet the former need, and an 

 amniotic sac the latter. 



407. Egg. — Three coverings are added to the egg cells as adaptations 

 to developm^ent in a non-aquatic environment (Fig. 255). These are 

 (1) a layer of alhumen, which provides protection against drying and 

 mechanical injury and also serves as food for the embryo; (2) an egg 

 membrane, which in some cases becomes thick and leathery and to which 

 may be added lime; and (3) a shell, present in many reptiles and normally 

 in all birds, composed entirely of lime. The albumen, the membrane, 

 and the shell are all secreted by glands lying along the course of the 

 oviduct in the order in which the envelopes which they form are added. 



vitelline 



membrane 



Germinal area 



Chalaza 



Chalaza 



Albumen 

 ^Egg shell 



Egg membrane 



White YeJIo w yolk 



yolk 



Fig. 255. — Diagrammatic section of a hen's egg. 



408. Amnion. — Since the eggs of reptiles and birds are meroblastic, 

 discoidal cleavage occurs and a sheet of cells, the blastoderm, is formed. 

 From a part of the blastoderm is developed the embryo, and the rest 

 of it grows around and completely envelops the yolk. The blastoderm 

 splits, forming two layers, ectoderm outside and entoderm inside, next 

 to the yolk. Between these appears a third layer, the mesoderm, and 

 this also splits into two layers, one of which, the somatic layer, lies next 

 to the ectoderm and with it forms the somatopleure, the other, the 

 splanchnic layer, next to the entoderm and with it forms the splanch- 

 nopleure. Between these two mesodermal layers is the coelom (Fig. 

 256 B). The amnion is a fold of the blastoderm outside the area forming 

 the embryo and is composed of two layers (Fig. 256 C), ectoderm and 

 somatic mesoderm, or somatopleure. As this fold grows up around the 

 embryo it meets above and incloses a sac, known as the amniotic sac, 

 which surrounds the embryo and which becomes filled with a watery 

 liquid known as the amniotic fluid (Fig. 256 D). The embryo is free 

 to move in this sac and thus is protected from jar, though the eggs may 



